[Qemu-devel] Re: [PATCH] qemu-kvm: Speed up of the dirty-bitmap-traveling

[Avi Kivity]

On 02/09/2010 11:54 AM, OHMURA Kei wrote:
> Thank you for your comments.  We have implemented the code which applied your 
> comments.  This is patch for qemu-kvm.c.
>   

Please reuse the changelog when reposing a patch, this makes it easier
for me to apply it.

> @@ -2438,27 +2438,34 @@ static int kvm_get_dirty_pages_log_range(unsigned 
> long start_addr,
>                                           unsigned long offset,
>                                           unsigned long mem_size)
>  {
> -    unsigned int i, j, n = 0;
> +    unsigned int i, j, k, start, end;
>      unsigned char c;
>      unsigned long page_number, addr, addr1;
>      ram_addr_t ram_addr;
> -    unsigned int len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
> +    unsigned int len = ((mem_size / TARGET_PAGE_SIZE) + TARGET_LONG_BITS - 
> 1) /
> +        TARGET_LONG_BITS;
> +    unsigned long *bitmap_ul = (unsigned long *)bitmap;
>  
>      /* 
>       * bitmap-traveling is faster than memory-traveling (for addr...) 
>       * especially when most of the memory is not dirty.
>       */
>      for (i = 0; i < len; i++) {
> -        c = bitmap[i];
> -        while (c > 0) {
> -            j = ffsl(c) - 1;
> -            c &= ~(1u << j);
> -            page_number = i * 8 + j;
> -            addr1 = page_number * TARGET_PAGE_SIZE;
> -            addr = offset + addr1;
> -            ram_addr = cpu_get_physical_page_desc(addr);
> -            cpu_physical_memory_set_dirty(ram_addr);
> -            n++;
> +        if (bitmap_ul[i] != 0) {
> +            start = i * TARGET_LONG_SIZE;
> +            end = (i + 1) * TARGET_LONG_SIZE;
>   

Should be a host long size, not guest. This will fail when running a
32-bit qemu-system-x86_64 binary.

> +            for (j = start; j < end; j++) {
> +                c = bitmap[j];
> +                while (c > 0) {
> +                    k = ffsl(c) - 1;
> +                    c &= ~(1u << k);
> +                    page_number = j * 8 + k;
> +                    addr1 = page_number * TARGET_PAGE_SIZE;
> +                    addr = offset + addr1;
> +                    ram_addr = cpu_get_physical_page_desc(addr);
> +                    cpu_physical_memory_set_dirty(ram_addr);
> +                }
> +            }
>   

Instead of using a nested loop if bitmap_ul[i] != 0, it is possible to
use just a single loop (while (c > 0)), and process a long's worth of data.

The only trickery is with big endian hosts, where the conversion from
bit number to page number is a bit complicated.

If we do this, we can convert the bitmap's type to unsigned long
throughout, and avoid the casts.

-- 
error compiling committee.c: too many arguments to function